The invention relates to an electronic trip device comprising detection means for detecting the current flowing in a conductor designed to be protected by an electrical circuit breaker associated with the trip device, an electronic processing unit performing at least a delayed tripping function of the long delay type, the assembly, connected to the detection means, comprising means for producing a value representative of the current, means for producing a first quantity, representative of the thermal state of the circuit breaker, the variations of which are proportional to the difference between the square of the value representative of the current and the square of a preset current threshold, and means for comparing the first quantity to a preset first threshold.
In most state-of-the-art electronic trip devices, the time delay of the long delay function is inversely proportional to the square of the fault current. These trip devices also take account of a possible decrease of the current, dropping below the current threshold, during the time delay. In a conventional manner (FIG. 1), when the value I representative of the current is greater than the current threshold Is, the quantity TLR representative of the thermal state of the circuit breaker, initially set to a minimum value, 0 in the figure, increases proportionally to the square of the current, simulating heating, whereas, when the value I representative of the current is lower than the current threshold Is, the quantity TLR decreases exponentially with time, simulating cooling. A tripping signal is produced when the quantity TLR exceeds a preset maximum value TLRmax.
It can be noted that, in some cases, a tripping signal is produced, although the mean value of the value representative of the current, during the time delay period, remains lower than the tripping threshold. This has, for the user, the same consequences as a decrease of the tripping threshold.